Chromium boride coated articles

ABSTRACT

A MATERIAL HAVING AN OUTER LAYER OF CHROMIUM BORIDE FORMED ON AN INTERMEDIATE SUBLAYER OF ESSENTIALLY PURE CHROMIUM ON A SUBSTRATE. THE CHROMIUM LAYER AND THE DIFFUSION ZONE BETWEEN THE CHROMIUM AND SUBSTRATE SERVE AS A THERMAL EXPANSION MISMATCH ACCOMMODATION REGION TO MINIMIZE CRACKING OF THE CHROMIUM BORIDE LAYER. THE CHROMIUM LAYER SERVES AS A SECONDARY CORROSION BARRIER AT THE BASE OF MINOR CRACKS SOMETIMES FORMED IN THE OUTER CHROMIUM BORIDE LAYER. IN THE PREFERRED EMBODIMENTS WE PRODUCE A CRACK-FREE CHROMIUM BORIDE LAYER BY SELECTING A SUBSTRATE MATERIAL WHICH IS EXPANSION MATCHED THEREWITH. THE PRESENT MATERIALS HAVE PARTICULAR UTILITY ON TEMPERATURE SENSING DEVICES IN MOLTEN ALUMINUM.

3,712,798 (THROMTUM BORRDE (DATED ARTTQLES Ray 5. Van Thyne, Oak Lawn, and John J. Rausch,

Antioch, ill, assignors to Surface Technology Corporation, Stone iark, Ill. No Drawing. Continuation-impart of application Ser. No. 671,?i89, Sept. 28, 1967. This appiieation Jan. 6, 1970,

Ser. No. 1,054

lint. Cl. C23c 11/08 US. til. 29-4195 A 5 llaims ABSTRACT OF THE DISCLOSURE A material having an outer layer of chromium boride formed on an intermediate sublayer of essentially pure chromium on a substrate. The chromium layer and the diffusion zone between the chromium and substrate serve as a thermal expansion mismatch accommodation region to minimize cracking of the chromium boride layer. The chromium layer serves as a secondary corrosion barrier at the base of minor cracks sometimes formed in the outer chromium boride layer. In the preferred embodiments we produce a crack-free chromium boride layer by selecting a substrate material which is expansion matched therewith. The present materials have particular utility on temperature sensing devices in molten aluminum.

REFERENCE TO PRlOR APPLICATION This patent application is a continuation-in-part of our copending application Ser. No. 671,189 filed Sept. 28, 1967, now abandoned.

BACKGROUND OF THE INVENTION Our invention is directed to a material which is essentially a tricomponent member having namely, a substrate, a chromium layer adherent thereto and a chromium boride reaction zone formed on the chromium. By so providing we find considerable improvement in the wear or abrasion resistance and corrosion resistance of such substrate materials.

In the practice of our invention it is critical that the chromium intermediate layer be used. Depending on the needs of the user, such layer may be partially or completely boronized to form chromium boride. We find it preferable, however, that the chromium be only partially boronized. For reasons set out below it is of key importance in our present invention that such chromium layer be formed and used as an independent member and not as a mere diffusion zone on the surface of the substrate.

The most pertinent prior art hereto of which we are aware is the patent to Samuel et al., US. 3,029,162, Process for the Production of Metallic Borides on the Surface of Metals. This patent teaches among others, the dilfusion of chromium into a metal substrate followed by the boronizing of the diffused layer. We have found, as is hereinafter set forth, that the mere diffusion layer concept suffers various serious shortcomings and that there is a substantial improvement in properties when the definite chromium interlayer is used as herein taught. If the substrate is iron we form pure chromium boride rather than a boride of mixed chromium-iron.

Accordingly, a principal object of our invention is to provide a new article of manufacture consisting essentially of a substrate having a chromium layer formed thereon which chromium layer is preferrably surface boronized, or completely boronized, to provide a chromium boride protective coating on said substrate.

Another object of our invention is to provide a chro- States Patent O hoe mium boride coating on a substrate wherein both coating and substrate are closely thermal expansion matched.

A further object of our invention is to provide a chromium boride coating on a chromium interlayer afiixed to a substrate wherein there is provided a thermal expansion mismatch accommodation region to minimize cracking of the chromium boride layer.

These, and other objects, features and advantages of our invention will become apparent to these skilled in this particular art from the following detailed disclosure thereof.

DESCRIPTION OF THE INVENTION In the practice of our invention the substrate is first coated with a layer of chromium. Such layer may be deposited by various methods known to those skilled in the art such as electroplating, physical or chemical vapor deposition, and others. The thickness of the chromium layer may be varied to accomplish specific objectives as later described herein, but it is essential that there be a discrete chromium layer and not merely, for example, the interdiffusion of the chromium into the substrate. The latter leads to the subsequent formation of mixed borides which we avoid.

The chromium layer is then surface impregnated with boron at elevated temperature to form a coating thereon consisting essentially of chromium boride. In most instances it is desirable to have a residual layer of unreacted chromium beneath the chromium boride layer, i.e. between the chromium boride and the substrate. Such chromium has excellent corrosion resistance in a variety of environments, and thus acts as a secondary corrosion barrier if cracks exist in the outer boride which penetrate into the chromium region.

In one embodiment of our invention we find it desirable to produce a crack-free chromium boride layer on certain substrate materials. To accomplish this it is necessary to use a substrate material with a coefiicient of thermal expansion closely matched to that of the chromium boride. We also find that materials with somewhat less favorably matched expansion behavior can be used to produce a crack-free chromium boride layer by controlling the relative thicknesses of the chromium boride, the residual chromium, and the chromium-substrate interditfusion zone. The crack-free chromium boride coatings have particular utility protecting substrate materials from corrosion in molten aluminum. We have also found that such chromium boride coated articles have adequate oxidation resistance at temperatures above the melting point of aluminum such that they may be used as immersion devices in which both resistance to attack by air and molten aluminum is necessary.

The following examples will demonstrate the importance of expansion matching to produce a crack-free chromium boride layer:

Specimens of AISI type 1020 and 4130 steel, type 310, 410, 430 and 446 stainless steel, and an alloy of were electroplated with chromium to thicknesses of 0.8 to 1.5 mils. These were subsequently surface impregnated with boron in a fused mixture of 60 weight percent Na B O 40 weight percent B 0. It will of course be understood that other boronizing treatments could also be employed. The boronizing was carried out at 1900 to 1950 F. for two to four hours. Lower temperatures may be used if a longer time is provided to produce boride diffusion coatings of comparable thickness. Also, we note that the presence of the chromium plate on ferrous materials permits boronizing at much higher temperatures since the eutectic melting interaction that normally occurs between iron-base alloys and boron at approximately 2000 F. is avoided. The chromium-boron eutectic is at 2850 F. F. A. Shunk, Constitution of Binary AlloysSecond Supplement, McGraw-I-Iill, New York, NY.

After boronizing as aforesaid, the chromium boride coating formed on the chromium plated 1020, 4130 and type 310 stainless was cracked with the extent of cracking being most severe on the austenitic stainless steel. These specimens were then immersed in commercial purity aluminum at a temperature of l300 to 1400 F. for one hour. After this exposure, a dendritic growth was observed on all of the specimens which simulated the crack pattern that existed in the chromium boride coating. This dendritic growth, formed in the boride cracks was composed of aluminides of chromium or of chromium and iron. After four hours of exposure to molten aluminum there was rapid disintegration of the coating.

On the other hand, the chromium boride layer formed on the chromium plated 400 series stainless steels and Fe-25-Cr5-Al alloy was crack-free. These alloys are essentially ferritic from room temperature to 1900 F., and thus have no transformation strains upon cooling, and have a close thermal expansion match with chromium boride. When exposed to molten aluminum at 1300 1400 F. the chromium boride layers on chromium plated type 410 and 430 stainless steels and on the alloy afforded complete protection of the underlying substrate. Specimens of chromium boride on chromium plated type 446 stainless steel made as herein taught were exposed for over 100 hours in molten aluminum without evidence of corrosion. Other materials such as the commercial nickel base alloy Mar M200 which is reported to have an expansion coefiicient about 10% greater than the chromium containing ferritic steels should also be an acceptable substrate material for use herein.

We also note that if minor defects occur in the chromium boride layer, due to prolonged exposure or as the result of improper chromium plating or boronizing, the resulting corrosion is of a minor nature being limited to pinpoint defects which grow relatively slowly with time. Such defect tolerance can be usefully employed, particularly if the underlying materials are not rapidly attacked by molten aluminum. We have found such defect tolerance with the chromium boride coating on chromium plated ferritic stainless steel clad on A1 ceramic. The defect tolerance also makes practical multiple chromium boride layer devices in which each layer provides a finite life; the total life of such device being considerable since such random defects are not likely to superimpose on each other.

We have found that if the alloys described above are simply boronized without prior chromium plating they have very poor corrosion resistance in molten aluminum. This is true even if the boride layers formed are relatively crack-free.

As previously stated, we have found that cracking of the chromium boride layer can be minimized by controlling the relative thicknesses of the boride, the residual chromium, and chromium-substrate interdiffusion zones. We produced a crack-free chromium boride layer on Armco iron, i.e. pure iron, by initially electroplating such specimen with 4 mils of chromium and subsequently boronizing at 1950 F. for four hours. After this treatment the chromium boride layer, which was crack-free, had a thickness of 0.9 mil the residual electroplated chromium layer was 3 mils thick, and the interdifiusion zone between chromium and Armco iron had a thickness of 7 mils. The latter two zones provided an expansion mismatch accommodation region between the chromium boride and Armco iron. The iron undergoes an allotropic transformation at 1670 F. which on heating results in a decrease in volume. Above this temperature the austenitic form of iron has a higher expansion coefiicient than chromium boride.

This same expansion mismatch accommodation region can be usefully employed on other materials to avoid or minimize cracking of the chromium boride layer. We note however that with increasing carbon content in an ironbase substrate material the extent of the chromium-substrate interditfusion zone is markedly restricted. A chromium boride coating of 0.6 mil thickness, was produced on a 1018 steel substrate which had been electroplated with 3 mils of chromium. Boronizing was carried out at 1950 F. for two hours in the previously described fused mixture. The residual chromium layer was 2.2 mils thick and the interdifiusion zone between the chromium and substrate had a thickness of 0.5 mil. The chromium boride layer contained slight cracks normal to the surface, which extended to the residual chromium layer. The cracks were very narrow and spaced about 7 mils apart. Such minor cracks are not expected to significantly detract from the utility of the chromium boride layer as a wear or abrasion resistant surface.

Furthermore, we find that the chromium boride layer has a hardness in excess of 3000 Diamond Pyramid Numerals (DPN) as measured by SO-gram load microhardness readings on a metallographically polished cross-section of such layer. This is significantly harder than the readings obtained on 1018 steel in which iron boride layers are formed. Such readings normally average 2200 DPN when measured by the above technique. The hardness of the chromium boride layer is also significantly higher than those obtained on boronized Fe-Cr alloys containing from 20-25% chromium. The hardness of the Fe-Cr layers on such materials averages approximately 2400 DPN. The hardness of the chromium boride layer is considerably greater than that obtained for carburized chromium plated materials or carburized stainless steels.

Others have previously boronized stainless steelsee Boronizing of High-Alloy Steels, Yu. M. Lakhtin and M. A. Pchelkina. Metallovedenie 1 Term. Obrab. Metallov, March 1961, No. 3, pp. 27-30 (Brutcher Translation No. 5660), and as noted above the patent regarding prechrornized steel. Since the latter results in the formation of a ferrous-base, chromium containing surface layer, boronizing of either material yields a coating that is different and which has substantially decreased properties over the present materials. Such mixed boride coatings are softer and less corrosion resistant than chromium boride. As part of our work, samples were given an intermediate anneal after plating with chromium but prior to boronizing and some limited interditfusion might also be obtained under certain pyrolytic chromium deposition methods. Such materials fall within the practice of our invention if interdiifusion is very limited so that the resulting coating is essentially chromium boride.

We also note that the chromium boride layer formed on chromium has a very smooth external surface resembling the original surface finish of the chromium deposit. Such surface is much smoother than that obtainable when iron or most iron-base alloys are boronized. This very hard chromium boride layer has utility for a variety of products that require wear and abrasion resistance, such as pump components, shears, scrapers, gripper dogs and chucks, engine components such as piston rings, cams, stems, and rods, chains and sprockets, gears, impellers, knife, razor edges, and slitters, glass cutters, saws, chain saw cutters, routers drills and taps. In addition, the surface finish can be controlled by, for example, varying the chromium plated finish prior to boronizing which is of considerable importance for applications such as dies, thread and wire guides, capstans, thread, plug and ring gages, mechanical seals such as rotary seals and valve components, forming rolls, cylinder liners, nozzles, bearings and bushings.

We also note that the underlying chromium layer affords useful corrosion protectionin environments less aggressive than molten aluminum-at the base of minor cracks that could exist in the boride. A specimen of 1018 steel, electroplated with chromium and boronized as described above, was exposed in a cabinet at 95 F. to a salt spray solution composed of NaCl in deionized water. After 60 hours of exposure there was no evidence of rusting or corrosion of the chromium or chromium boride layers. This offers significant advantage over the iron boride that forms without the chromium interlayer and which exhibits significant rusting. Furthermore, the chromium boride layer has excellent corrosion resistance in strong acids. Thus these materials have further utility for use in the wear and abrasion resistant applications previously described, when such applications further involve use in a corrosive environment.

The thickness of the chromium deposit prior to boronizing can be varied over wide limits in the practice of our invention. It is only essential that the thickness be adequate to provide an outer surface layer which is essentially chromium boride having its excellent corrosion resistance and high hardness. For practical purposes the chromium layer can be as thin as 0.1 mil. We are aware of no practical limitation on the maximum thickness of chromium. However, the difiiculty of fabricating chromium metal to shape by methods other than deposition techniques is well known. Thus, the use of chromium as an interlayer on another structural material is of obvious advantage.

It will be understood that various modifications and variations may be eflfected without departing from the spirit or scope of the novel concepts of our invention.

We claim as our invention:

1. A tricomponent article of manufacture consisting essentially of:

(a) a ferrous base substrate member;

(b) a discrete chromium layer aflixed to said substrate member;

(0) a chromium boride surface zone on said chromium layer; which article is characterized by having excellent corrosion resistance.

2. The article as defined in claim 1 wherein said sub strate is an alloy steel, essentially ferritic at temperatures from room temperature to at least 1900 F., and having a thermal expansion coefiicient closely matched to that of chromium boride.

3. The article as defined in claim 2 wherein said article is in the form of a temperature sensing device.

4. The article as defined in claim 1 wherein said substrate member is iron.

5 The article as defined in claim 1 wherein said substrate member is an alloy steel which is essentially ferritic at temperatures from room temperature to at least 1900 F.

References Cited UNITED STATES PATENTS 2,048,276 7/1936 Marlies el: al. 29-195 A 3,029,162 4/1962 Samuel et al. 148-63 2,874,065 2/1959 Herz et al 117-105 X 3,018,194 1/1962 Norman et al. 117-107.2 R

FOREIGN PATENTS 1,078,095 11/1960 Germany 148-63 OTHER REFERENCES Zhunkovskii et al., Fiz. Khim. Obrab. Mater. 1968 (4) 132-40, chem. Abstracts :6184e, Jan. 13, 1969.

RALPH S. KENDALL, Primary Examiner U.S. Cl. X.R. 

